Iím planning on buying a Mac Mini as a media center (with Plex for the movies and itunes for the music), but I have some questions about the best option for additional storage.

I need at least 2TB of storage for all of my media files, and would like the possibility of easy expansion. I would also like to have a backup of all of the files.

I would like to be able to download files from the Internet onto my MacBook and wirelessly drag-and-drop them to wherever the media is stored. I'd also like to wirelessly stream the video files to an iPad and an iPhone via AirVideo for example.

From what I gather, my two options are external hard drives hooked up directly to the Mini, or a NAS setup--except I've researched a whole bunch, and I still have no idea if one is a better option than the other for what my goals are. (What would be the point of a NAS if my mac mini would be dedicated to the media center ?)

Some information that may or may not be relevant:
- All the computers in my household are Macs running OS X Lion and Mountain Lion
- The Mac Mini would be in the ground floor and plugged to my DSL modem, a videoprojector and an AV Amplifier (would it be possible to watch movies on my lcd TV screen in the first floor ?)
- I'm okay with doing manual backups once a week or so of my media drive--constant synchronization is not a necessity but would be the icing on the cake

I have a quad drive FW case, and it's nice, but it has to be near the mini.

With a NAS you can put it anywhere. If you get a NAS that can run Plex then it can be your Plex server. I would think that iTunes would be able to handle having it's library on a network drive. I would check the Plex forums to see which NAS would be the easiest to set up, and would work the best.

So if I had to do it again, I'd go with a quad drive NAS, rather than immediately local storage.

A NAS, by definition, has little benefit unless you want to use it in a (possibly heterogeneous) multi-computers environment without a central, always running, server.
If your Mac Mini is always running, you can do as much (actually more) accessing its shared Firewire/USB storage than with a NAS.
And you would not have to learn how to administrate a NAS...

Note also that a WiFi/Network connection may be subject to disturbances which could generate images freezes when playing back.

Based on your requirements, I would go for a RAID1 (= mirror) 2 disks enclosure, which at present would give you a maximum of 4TB capacity.
Mac OS X natively supports RAID1 (format through Disk Utility).
If you want a footprint compatible with the latest Mac Minis, you can buy 2 single 3"5 disk enclosures such as NewerTech USB3 miniStack, or one such as Stardom SC2-WB3 Safe Capsule.
Note that the Stardom could also be directly connected to an Apple Airport Extreme WiFi router if you have one, making it the equivalent of a multi-functions Time Capsule... or a NAS

If RAID1 is not enough for your "backup" needs, you may in addition use a backup utility on top of it (with 4TB you have some margin to backup 2TB).

I would not go for a true hardware RAID solution (Synology, Drobo, Qnap, ...), directly attached or WiFi attached, unless you are sure you need (much) more capacity than a single 3"5 disk can offer.
RAIDs are more expensive, with complex failure symptoms, and need to be administrated, not always in a "Mac way".

How many power outlets do you have? How many outlets do you want to keep plugging wallworts into? Are you never going to add more to your media collection? Are you ever going to get a Roku, or ATV, or a 'Smart' TV?

I use external LaCie drives with Firewire 800/400 that are directly attached to my Mac mini. It's just the easiest way to go. I also think that it offers the best performance, while eliminating some of the traffic over your home network. Plus, they can still feed other computers on your network using the Mac mini as a media server._________________My Setup